The present invention relates to a gas pressure gauge or a vacuum gauge used broadly in vacuum devices and instruments, and more specifically relates to a vacuum gauge of the thermo-conductive type utilizing a temperature dependant quartz oscillator operative based on the fact that the thermo-conductivity of gas is a function of its pressure or degree of its density.
A thermo-conductive type vacuum gauge such as a Pirani gauge has been generally used for the pressure measurement in intermediate and low pressure range (10.sup.-3 -1 torr). FIG. 2 shows an example of the conventional vacuum gauge of the thermo-conductive type with digital display. One resistive element of a bridge circuit is composed of a filament 11 which is disposed within a vacuum chamber and operates as a gas pressure sensor. The other resistive elements 11a, 11b and 11c of the bridge circuit are disposed outside the vacuum chamber, and a DC power source 12 applies a constant DC voltage to the bridge circuit. The equilibrium voltage of the bridge circuit is amplified by a DC amplifier 13, and the amplified DC voltage is converted into a corresponding digital signal by an analog-to-digital converter 14 (A/D converter). A counter 15 counts the output signal of A/D converter 14 to produce an address signal for a subsequent read-only-memory 16 (ROM). The ROM 16 feeds to a decoder 17 a display signal (i.e., the signal indicative of the value of gas pressure) corresponding to the output signal of the counter 15 (i.e., the signal representative of gas pressure information), and the decoder 17 decodes the display signal so that a subsequent display device 18 (for example, a seven-digit LED display device) can display the value of gas pressure. The logic system from the A/D converter to the decoder is controlled by a clock signal which is fed from a clock generater 19 having an oscillating source in the form of a quartz oscillator 20.
In operation, if the pressure within the vacuum chamber decreases, the heat dissipating from the filament 11 decreases to increase the temperature of the filament 11 and therefore to increase the electric resistance of the filament so that the equilibrium voltage of the bridge circuit is increased. As described above, the equilibrium voltage of the bridge circuit is a function of the gas pressure within the vacuum chamber, hence the pressure within the vacuum chamber can be detected by measuring the equilibrium voltage of the bridge circuit.
Though the conventional vacuum gauge of the thermoconductive type can be conveniently and widely used to measure the gas pressure in the intermediate and low pressure ranges, it has the following drawbacks. 1. Since the drift of a DC amplifier would directly cause a measurement error, a highly stable (and therefore expensive) DC amplifier is required. 2. An expensive A/D converter is required for the digital display of the measurement result. 3. Since the filament generates increasingly more heat as the gas pressure decreases, the pressure sensor deteriorates through use.
As described above, the conventional vacuum gauge of the thermo-conductive type has drawbacks such as the need for a high grade DC amplifier and the need for an expensive A/D converter in case of the digital display of the measurement result, both of which increase the cost of the vacuum gauge, and the deterioration of the pressure sensor.